Fuel reforming system and control method of fuel reforming system

ABSTRACT

A fuel reforming system includes a reformer that produces fuel gas that includes hydrogen by reforming a reforming fuel by a steam reforming reaction using reforming water, and; supply amount regulating means for regulating an amount of the reforming water supplied to the reformer; pressure detecting means for detecting a gas pressure in the reformer; and correcting means for correcting the amount of supplied reforming water with the supply amount regulating means based on a fluctuation amount in the gas pressure detected by the pressure detecting means.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a fuel reforming system and a control method of a fuel reforming system.

2. Description of the Related Art

Hydrogen that is consumed by a fuel cell or the like is able to be produced in a fuel reforming system. In a fuel reforming system, hydrogen is produced by a steam reforming reaction between reforming water and a reforming fuel such as hydrocarbon that takes place in a reformer. The steam reforming reaction is a chemical reaction process, so if the amount of reforming water that is supplied to the reformer becomes unstable, the amount of produced hydrogen also becomes unstable. Technology described in Japanese Patent Application Publication No. 6-176787 (JP-A-6-176787) therefore controls the amount of reforming water that is supplied by providing a steam flow meter.

However, steam flow meters are extremely expensive, so the cost ends up increasing.

SUMMARY OF THE INVENTION

The invention provides a fuel reforming system, and a control method of a fuel reforming system, capable of optimally controlling the supply amount of reforming water without using a detector such as a flow meter that detects the steam flowrate.

A first aspect of the invention relates to a fuel reforming system. This fuel reforming system includes a reformer that produces fuel gas that includes hydrogen by reforming a reforming fuel by a steam reforming reaction using reforming water, and; supply amount regulating means for regulating an amount of the reforming water supplied to the reformer; pressure detecting means for detecting a gas pressure in the reformer; and correcting means for correcting the amount of supplied reforming water with the supply amount regulating means based on a fluctuation amount in the gas pressure detected by the pressure detecting means.

According to this aspect, the reforming water supply amount is corrected based on the amount of fluctuation in the gas pressure. Therefore, the amount of supplied reforming water can be optimally controlled without using an expensive detector such as a flow meter that detects the steam flowrate.

In the foregoing aspect, the correcting means may correct the amount of supplied reforming water with the supply amount regulating means based on a standard deviation of the gas pressure detected by the pressure detecting means.

According to this structure, the detection accuracy of the gas pressure fluctuation amount improves.

In the structure described above, the correcting means may perform a correction that decreases the amount of supplied reforming water, when the standard deviation of the gas pressure exceeds a first predetermined value.

Also in the structure described above, the correcting means may perform a correction that increases the amount of supplied reforming water, when the standard deviation of the gas pressure falls below a second predetermined value.

According to this structure, the reforming water supply amount is corrected such that the standard deviation of the gas pressure falls within a predetermined range. Therefore, the S/C ratio can be controlled to a desirable value.

In the structure described above, the correcting means may perform a correction that decreases the amount of supplied reforming water, when the standard deviation of the gas pressure exceeds a first predetermined value, or the correcting means may perform a correction that increases the amount of supplied reforming water, when the standard deviation of the gas pressure falls below a second predetermined value. Also, the first predetermined value may be larger than the second predetermined value.

In the structure described above, the first predetermined value and the second predetermined value may be determined by a ratio between a number of moles of the reforming water supplied to the reformer and a number of moles of carbon in the reforming fuel supplied to the reformer.

In the structure described above, the ratio of the number of moles of the reforming water to the number of moles of the carbon of the first predetermined value may be 3:1, and the ratio of the number of moles of the reforming water to the number of moles of the carbon of the second predetermined value may be 2:1.

In the structure described above, the correcting means may not correct the amount of supplied reforming water when the standard deviation of the gas pressure does not exceed the first predetermined value, and the standard deviation of the gas pressure does not fall below the second predetermined value.

In the structure described above, the correcting means may not correct the amount of supplied reforming water with the supply amount regulating means when the amount of fluctuation in a reforming amount of the reformer exceeds a third predetermined value.

According to this structure, an erroneous correction in a transient state in which the amount of fluctuation in the reforming amount of the reformer is large can be avoided.

The fuel reforming system described above may also include setting means for setting an amount of reforming fuel supplied to the reformer. Also, the correcting means may not correct the amount of supplied reforming water with the supply amount regulating means when a difference between a reforming water supply amount derived based on the amount of supplied reforming fuel set by the setting means and a reforming water supply amount derived based on the gas pressure detected by the pressure detecting means exceeds a fourth predetermined value.

According to this structure, an erroneous correction in a small anode pressure fluctuation region (i.e., a region with a small amount of pressure fluctuation) due to an excessive supply of reforming water can be avoided.

In the structure described above, a correction range of the reforming water supply amount corrected by the correcting means may be changed according to a reforming amount of the reformer.

A second aspect of the invention relates to a control method of a fuel reforming system that produces fuel gas that includes hydrogen by reforming a reforming fuel by a steam reforming reaction using the reforming water, and. This control method includes detecting a gas pressure in the reformer, and correcting the amount of supplied reforming water based on a fluctuation amount in the detected gas pressure.

According to this aspect, the reforming water supply amount is corrected based on the amount of fluctuation in the gas pressure. Therefore, the amount of supplied reforming water can be optimally controlled without using an expensive detector such as a flow meter that detects the steam flowrate.

In the structure described above, the amount of supplied reforming water may be corrected based on a standard deviation of the detected gas pressure.

According to this structure, the detection accuracy of the gas pressure fluctuation amount improves.

In the structure described above, a correction that decreases the amount of supplied reforming water may be performed when the standard deviation of the gas pressure exceeds a first predetermined value.

In the structure described above, a correction that increases the amount of supplied reforming water may be performed when the standard deviation of the gas pressure falls below a second predetermined value.

According to this structure, the reforming water supply amount is corrected such that the standard deviation of the gas pressure falls within a predetermined range. Therefore, the S/C ratio can be controlled to an appropriate value.

In the structure described above, a correction that decreases the amount of supplied reforming water may be performed when the standard deviation of the gas pressure exceeds a first predetermined value, or a correction that increases the amount of supplied reforming water may be performed when the standard deviation of the gas pressure falls below a second predetermined value. Also, the first predetermined value may be larger than the second predetermined value.

In the structure described above, the first predetermined value and the second predetermined value may be determined by a ratio between a number of moles of the reforming water supplied to the reformer and a number of moles of carbon in the reforming fuel supplied to the reformer.

In the structure described above, the ratio of the number of moles of the reforming water to the number of moles of the carbon of the first predetermined value may be 3:1, and the ratio of the number of moles of the reforming water to the number of moles of the carbon of the second predetermined value may be 2:1.

In the structure described above, the amount of supplied reforming water may not be corrected when the standard deviation of the gas pressure does not exceed the first predetermined value, and the standard deviation of the gas pressure does not fall below the second predetermined value.

In the structure described above, the amount of supplied reforming water may not be corrected when the amount of fluctuation in a reforming amount of the reformer exceeds a third predetermined value.

According to this structure, an erroneous correction in a transient state in which the amount of fluctuation in the reforming amount of the reformer is large can be avoided.

The control method described above may also include setting an amount of reforming fuel supplied to the reformer. Also, the amount of supplied reforming water may not be corrected when a difference between a reforming water supply amount derived based on the set amount of supplied reforming fuel and a reforming water supply amount derived based on the detected gas pressure exceeds a fourth predetermined value.

According to this structure, an erroneous correction in a small anode pressure fluctuation region due to an excessive supply of reforming water can be avoided.

In the structure described above, a correction range of the corrected reforming water supply amount may be changed according to a reforming amount of the reformer.

According to the structures described above, a fuel reforming system, and a control method of a fuel reforming system, capable of optimally controlling the supply amount of reforming water without using a detector such as a flow meter that detects the steam flowrate are each able to be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic diagram of the structure of a fuel cell system to which a fuel reforming system according to a first example embodiment of the invention is applied;

FIG. 2 is a partial perspective view that includes a cross section of a unit cell that forms part of a fuel cell stack;

FIG. 3 is a perspective view of the fuel cell stack;

FIG. 4 is a perspective view of only the fuel cell stack, a combustion portion, and a reformer;

FIG. 5 is a perspective view of the reformer in detail;

FIG. 6 is a graph of the relationship between saturated steam pressure and steam temperature;

FIG. 7 is a graph of the correlation between the reforming water supply amount and the gas pressure in a power generating test from a light load to a normal load (i.e., a rated load) of the fuel cell stack;

FIG. 8 is a view of a change in the gas pressure when the reforming water supply amount is gradually increased;

FIG. 9 is a view of the change in the gas pressure after reforming water starts to be supplied;

FIG. 10 is a flowchart illustrating a reforming water supply amount optimization routine when a reforming water flowrate control program is executed; and

FIG. 11 is a view of maps and a table used with the flowchart in FIG. 10.

DETAILED DESCRIPTION OF EMBODIMENTS

Example embodiments of the present invention will be described in greater detail below with reference to the accompanying drawings.

First Example Embodiment

FIG. 1 is a schematic diagram of the structure of a fuel cell system 1000 to which a fuel reforming system according to a first example embodiment of the invention is applied. The fuel reforming system 1000 includes a fuel cell stack 100, a combustion portion 200, a reformer 400, and a control portion 600 and the like.

The fuel cell stack 100 is able to obtain electromotive force by inducing an electrochemical reaction between hydrogen as the fuel gas and oxygen in the air as the oxidant gas, at each electrode. In this example embodiment, the fuel cell stack 100 is a solid oxide fuel cell (SOFC) with a reaction temperature of approximately 600° C. to 1000° C. Off-gas discharged from an anode side of the fuel cell stack 100 (hereinafter also referred to as “anode off-gas”) is supplied to the combustion portion 200 via an anode off-gas passageways 108.

An air supply system that supplies the air as the oxidant gas to the fuel cell stack 100 includes an air supply passageways 114 and an air pump 116 provided in the air supply passageways 114. The air pump 116 supplies air that has been drawn in from outside via an air cleaner to a cathode of the fuel cell stack 100 as the oxidant gas via the air supply passageways 114. Off-gas discharged from the cathode side of the fuel cell stack 100 (hereinafter also referred to as “cathode off-gas”) is supplied to the combustion portion 200 via a cathode off-gas passageways 118.

The combustion portion 200 is provided with a glow ignition mechanism, and combusts the anode off-gas supplied via the anode off-gas passageways 108 using the oxygen in the cathode off-gas supplied via the cathode off-gas passageways 118, by applying a predetermined voltage to the glow ignition mechanism. The combustion portion 200 is provided with a combustion off-gas passageways 202, and combustion off-gas that includes both combusted gas and uncombusted gas in the combustion portion 200 is discharged into the atmosphere via the temperature sensor 202.

A heat exchanger 300 is provided with a tap water introducing passageways 302 and a hot water discharge passageways 304. In the heat exchanger 300, tap water introduced via the tap water introducing passageways 302 is heated by the combustion heat given off by combustion in the combustion portion 200 to become hot water. The hot water discharge passageways 304 is connected to a water storage tank, not shown. The hot water heated by the heat exchanger 300 is stored in the water storage tank via the hot water discharge passageways 304. The water storage tank may be connected to a bathtub or a shower or the like in a home of the user, and the hot water stored in the water storage tank may be supplied to the user according to the needs of the user, for example. Incidentally, the hot water in the water storage tank may also be reintroduced into the heat exchanger 300 and reheated. For example, the hot water in the water storage tank may be reheated if the temperature of the hot water is lower than the temperature needed by the user, due to the temperature of the water decreasing.

The reformer 400 includes a mixing portion and a reforming portion. Reforming fuel supplied from a reforming fuel tank 402, which will be described later, and water (hereinafter also referred to as “reforming water”) supplied from a reforming water tank 500, that will also be described later, are mixed together in the mixing portion. The reforming water is vaporized in the mixing portion. Hereinafter, the mixed and vaporized gas in the mixing portion will be referred to as “mixed gas”. The reforming portion includes a reforming catalyst that promotes a reforming reaction. When the mixed gas produced in the mixing portion is introduced into the reforming portion, the reforming reaction progresses due to the reforming catalyst, such that fuel gas that includes hydrogen is produced. This reforming reaction is an endothermic reaction, so heat input is required. In this example embodiment, heat generated by the combustion reaction in the combustion portion 200 is used. The reforming catalyst is appropriately determined according to the reforming fuel used in the reforming reaction. Incidentally, other than hydrogen, the fuel gas produced in the reformer 400 and supplied to the fuel cell stack 100 includes carbon monoxide (CO), carbon dioxide (CO₂), and unreacted reforming fuel.

The reforming fuel supply system that supplies the reforming fuel to the reformer 400 includes the reforming fuel tank 402, a reforming fuel supply passageways 404, and a flow control valve 406 that is provided in the reforming fuel supply passageways 404. A pressure sensor 410 that detects the gas pressure in the reformer 400 (hereinafter also referred to as the “anode pressure”) is also provided in the reforming fuel supply system. The pressure sensor 410 may be provided at any location in the conduit from the flow control valve 406 to the anode of the fuel cell stack 100.

The reforming fuel tank 402 stores hydrocarbon as the reforming fuel. The reforming fuel is not limited to hydrocarbon. That is, ammonia or the like may also be used. Incidentally, the reforming fuel tank 402 does not have to be used. That is, a supply line for coal gas (town gas) or the like may also be used as reforming fuel supplying means. The hydrocarbon stored in the reforming fuel tank 402 is regulated to a predetermined flowrate by the flow control valve 406, and supplied to the reformer 400 via the reforming fuel supply passageways 404. The fuel gas that includes the hydrogen, the carbon monoxide, the carbon dioxide, and the unreacted reforming fuel produced by the reformer 400 is supplied to the anode of the fuel cell stack 100 via a fuel supply line 408.

A reforming water supply system for supplying reforming water to the reformer 400 includes a condenser 504, a circulation pump 505, a condensed water line 506, a reforming water tank 500, a reforming water supply line 508, a reforming water pump 510, and a reforming water metering electromagnetic valve 512. The condenser 504 is provided in an off-gas discharge line 206, and condenses water vapor in the combustion off-gas that has been cooled in the heat exchanger 300. The condensed water line 506 is connected to the condenser 504. The circulation pump 505 introduces the condensed water that has been condensed by the condenser 504 into the reforming water tank 500 via the condensed water line 506. The condensed water (i.e., the reforming water) stored in the reforming water tank 500 is introduced into the reforming fuel supply passageways 404 via the reforming water supply line 508 by the reforming water pump 510. The reforming water metering electromagnetic valve 512 regulates the amount of reforming water that is introduced into the reforming fuel supply passageways 404. In this way, both the reforming water and the hydrocarbon as the reforming fuel are supplied to the reformer 400.

The control portion 600 is configured as a logic circuit that is centered around a microcomputer. The control portion 600 includes a CPU 610 that executes predetermined calculations and the like, a reforming water flow control program 621, memory 620 in which a map 622, a map 623, a map 624 and the like are stored, and an input/output port 630 that inputs and outputs various signals. The CPU 610 executes the predetermined calculations and the like according to the reforming water flow control program 621.

The control portion 600 obtains information relating to detection signals from the pressure sensor 410 described above and a request for the fuel cell stack 100 to generate power and the like. The control portion 600 then calculates an appropriate flowrate for the reforming water to be supplied to the reformer 400 based on the obtained information, and outputs a drive signal to the reforming water metering electromagnetic valve 512. Also, the control portion 600 also outputs drive signals to various portions related to power generation by the fuel cell stack 100, such as the flow control valve 406 and the air pump 116.

Next, the fuel cell stack 100, the combustion portion 200, and the reformer 400 will be described in greater detail. FIG. 2 is a partial perspective view of a unit cell 10 which includes a cross-sectional view of the unit cell 10. As shown in FIG. 2, the unit cell 10 has a flattened column shape as a whole. Within an electroconductive support 11 having gas permeability, there are formed a plurality of fuel gas passageways 12 extending through the unit cell 10 in the direction of an axis thereof. An anode 13, a solid electrolyte 14 and an cathode 15 are stacked in that order on one of two side surfaces of an outer periphery of the electroconductive support 11. On the other side surface, there is provided an interconnector 17 underneath which a joining layer 16 lies. A p-type semiconductor layer 18 is provided on top of the interconnector 17. Concreatly, the anode 13 and the interconnector 17 face each other having the electroconductive support 11 in between.

A fuel gas containing hydrogen is supplied to the fuel gas passageways 12, so that hydrogen is supplied to the anode 13. On the other hand, an oxidant gas containing oxygen is supplied to the surroundings of the unit cell 10. Electricity is generated by the following electrode reactions occurring at the cathode 15 and the anode 13. The electricity generating reaction takes place at a temperature, for example, 600° C. to 1000° C.

-   Anode: ½O₂+2e⁻→O²⁻(solid electrolyte) -   Cathode: O²⁻(solid electrolyte)+H₂→H₂O+2e⁻

A material of the cathode 15 has oxidation resistance, and is porous so that gaseous oxygen will reach an interface between the cathode 15 and the solid electrolyte 14. The solid electrolyte 14 has a function of migrating oxygen ion O²⁻ from the cathode 15 to the anode 13. The solid electrolyte 14 is composed of an oxygen ion-conductive oxide. Besides, the solid electrolyte 14 is stable in an oxidative atmosphere and also in a reductive atmosphere, and is composed of a compact material, in order to physically separate the fuel gas and the oxidant gas. The anode 13 is formed from a porous material that is stable in the reductive atmosphere and has affinity to hydrogen. The interconnector 17 is provided in order to electrically connect the unit cells 10 to each other in series, and is composed of a compact material so as to physically separate the fuel gas and the oxygen-containing gas.

For example, the cathode 15 is formed from a lanthanum cobaltite-base perovskite-type composite oxide, and the like, that is highly conductive for both electrons and positive ions. The solid electrolyte 14 is formed from, for example, a zirconia (ZrO₂) containing Y₂O₃ (YSZ) which is high in ion conductivity, and the like. The anode 13 is formed from, for example, a mixture of Y₂O₃-containing ZrO₂ (YSZ) and Ni, which is high in electron conductivity. The interconnector 17 is formed from, for example, a solid solution of LaCrO₃ with an alkaline earth oxide. As for these materials, materials that are similar to each other in thermal expansion coefficient are used.

FIG. 3 is a perspective view of the fuel cell stack 100. As shown in FIG. 3, in the fuel cell stack 100, a plurality of unit cells 10 are stacked together via collector members. In this case, each unit cell 10 is stacked such that the anode 13 side and the cathode 15 side face one another. Incidentally, in FIG. 3, the thin arrows indicate the flow of fuel gas, and the bold arrows indicate the flow of oxidant gas.

FIG. 4 is a perspective view of only the fuel cell stack 100 and the reformer 400. As shown in FIG. 4, two of the fuel cell stacks 100 are arranged on a manifold 40, and the reformer 400 is arranged above the fuel cell stacks 100. Also, the combustion portion 200 is formed between an upper ends of the unit cells 10 and the reformer 400.

The two fuel cell stacks 100 are disposed side by side so that the stack direction of the unit cells 10 of the two fuel cell stacks 100 are substantially parallel to each other. The reformer 400 extends over one of the two fuel cell stacks 100 in the stack direction of the unit cells 10, extends over the other fuel cell stack 100 in the stack direction of the unit cells 10, and the two extended ends are interconnected to form substantially a U-shape. The outlet of the reformer 400 and the inlet of the manifold 40 of the anode 13 are connected by a fuel gas conduit 50.

The lower end of the fuel cell stack 100 is fixed to the manifold 40. Holes that are communicated with fuel gas passageways 12 of the unit cells 10 are formed in the manifold 40. Accordingly, a fuel gas flow path is formed that communicates the reformer 400 with the fuel gas passageways 12 via the fuel gas conduit 50 and the manifold 40.

FIG. 5 is a perspective view of the reformer 400 in detail. As shown in FIG. 5, the reformer 400 has a structure in which a vaporizing portion 31, a heating portion 32, and a reforming portion 33 are connected together in order from an inlet side. The vaporizing portion 31 is an empty portion that vaporizes reforming water using the combustion heat of the anode off-gas.

The heating portion 32 is a space in which the reforming water and the hydrocarbon-base fuel are heated by combustion heat of the fuel off-gas. For example, ceramics balls are enclosed in the heating portion 32. The reforming portion 33 is a space in which the steam-reforming reaction of the reforming water and the hydrocarbon-base fuel takes place. For example, ceramics balls to which a reforming catalyst, such as Ni, Ru, Rh, Pt, etc., is applied are enclosed in the reforming portion 33.

As shown in FIG. 4, the fuel gas is supplied to the fuel gas passageways 12 of the unit cells 10 from the manifold 40. The oxidant gas moves downward between the fuel cell stacks 100, after which it is supplied around the unit cells 10. Then power is generated in each of the unit cells 10.

Fuel gas (i.e., anode off-gas) that did not contribute to the generation of power in the unit cells 10 and oxidant gas (i.e., cathode off-gas) that did not contribute to the generation of power in the unit cells 10 combine at the upper ends of the unit cells 10. In this example embodiment, the portion where anode off-gas combusts between the upper ends of the unit cells 10 and the reformer 400 functions as the combustion portion 200.

Continuing on, the behavior of the reforming water will be described with reference to FIGS. 6 to 9. Of the raw materials supplied to the reformer 400, that which changes from a liquid phase to a gas phase is the reforming water. FIG. 6 is a graph of the relationship between the saturated steam pressure and the steam temperature. In FIG. 6, the horizontal axis represents the steam temperature, and the vertical axis represents the saturated steam pressure. As shown in FIG. 6, the saturated steam pressure sharply rises as the steam temperature increases. Reforming water evaporates more easily under a high saturated steam pressure by the reformer 400 being heated by the combustion portion 200. Therefore, assuming that the steam temperature is constant, in a closed space such as the fuel supply line 408, the reforming water flowrate dictates the anode pressure.

FIG. 7 is a graph of the correlation between the reforming water supply amount and the anode pressure in a power generating test from a light load to a normal load (i.e., a rated load) of the fuel cell stack 100. In FIG. 7, the horizontal axis represents the amount of reforming water supplied to the fuel cell stack 100, and the vertical axis represents the anode pressure. As shown in FIG. 7, a positive correlation can be seen between the reforming water supply amount and the anode pressure. From the results in FIG. 7, with the anode pressure, the pressure generated by the vaporization of the reforming water is dominant. Therefore, the reforming water flowrate can be estimated based on the anode pressure.

When the reforming water vaporizes, bumping occurs. Bumping is a phenomenon in which gas that has been dissolved in a liquid instantaneously expands. In this example embodiment, when the reforming water bumps, the anode pressure sharply increases. When the amount of heat applied to vaporize the reforming water is controlled to be substantially constant, such as when power is steadily being generated by the fuel cell stack 100, the steam temperature may be regarded as being substantially constant. Therefore, if the amount of heat applied is set such that bumping occurs when reforming water of a predetermined flowrate is supplied, it is possible to detect whether the amount of reforming water supplied is appropriate by detecting the fluctuation range of the anode pressure.

FIG. 8 is a view of a change in the anode pressure when the reforming water supply amount is gradually increased. In FIG. 8, the horizontal axis represents the elapsed time, and the vertical axis represents the S/C ratio, the anode pressure, the anode pressure fluctuation index, and the combustion portion heat quantity. Incidentally, the value of the S/C ratio is represented by the vertical axis on the right side. The “S” of the S/C ratio represents the number of moles of reforming water supplied to the reformer 400, and the “C” represents the number of moles of carbon in the reforming fuel supplied to the reformer 400. Therefore, when the S/C ratio is high, the amount of reforming water supplied to the reformer 400 is large, and when the S/C ratio is low, the amount of reforming water supplied to the reformer 400 is small. The equivalent weight of the S/C ratio of the steam reforming reaction is 1. However, when taking into account control variation and reforming fuel flowrate distribution variation and the like, the control target value of the S/C ratio may be approximately 2 to 3. The anode pressure fluctuation index is indicated by the standard deviation of the difference between the last measured anode pressure and the current measured anode pressure (sample number n=50 in FIG. 8). The combustion portion heat quantity is the combustion heat of the combustion portion 200.

As shown in FIG. 8, if the S/C ratio is small, the anode pressure fluctuation index will also be small. If the S/C ratio is larger than a predetermined value, the anode pressure fluctuation index will be equal to or greater than a predetermined value. Also, if the S/C ratio becomes larger than a predetermined value, the anode pressure fluctuation index will sharply decrease. It is therefore evident that bumping occurs when the S/C ratio is within a predetermined value range (i.e., when the reforming water supply amount is within a predetermined range). On the other hand, it is evident that bumping does not occur when the S/C ratio is smaller than a predetermined value (i.e., when the reforming water supply amount is small) and when the S/C ratio is larger than a predetermined value (i.e., when the reforming water supply amount is excessive). Thus, bumping will not occur when the amount of heat applied per unit of reforming water is too small or too large. Therefore, if the pressure fluctuation of the anode pressure is within a predetermined range, the steam temperature can be controlled to be substantially constant.

FIG. 9 is a view of the change in the anode pressure after reforming water starts to be supplied. In FIG. 9, the horizontal axis represents the elapsed time and the vertical axis represents the reforming water supply amount, the anode pressure, and the anode pressure fluctuation index. As shown in FIG. 9, when a large fluctuation does not occur in the anode pressure, the anode pressure fluctuation index is small. In contrast, if the anode pressure fluctuates unexpectedly over time, the anode pressure fluctuation index will become larger. From the results shown in FIG. 9, it can be assumed that bumping is the main cause of fluctuation in the anode pressure.

Here, the behavior of the reforming water illustrated in FIGS. 6 to 9 will be summarized. First, the reforming water is vaporized in the reformer 400. As this happens, the anode pressure changes. This anode pressure depends on the amount of reforming water supplied. Also, fluctuation in the anode pressure is caused by bumping of the reforming water. Therefore, the amount of reforming water supplied can be optimized by detecting bumping of the reforming water from the anode pressure.

Depending on the installation environment of the fuel cell stack 100, the range of the S/C ratio being 2 to 3 may overlap with the range in which bumping occurs. Therefore, if an attempt is made to control the S/C ratio to the appropriate range, the reforming water supply amount can be corrected such that bumping occurs, so the reforming water supply amount can be corrected to an appropriate value. In this example embodiment, optimum control of the reforming water supply amount using the pressure sensor 410 that detects the anode pressure will be described.

FIG. 10 is a flowchart illustrating a reforming water supply amount optimization routine when the reforming water flow control program 621 is executed in the CPU 610 of the control portion 600. This routine is executed in predetermined cycles (such as at one second intervals) after the fuel reforming system 1000 is activated.

As shown in FIG. 10, the CPU 610 obtains a request for the fuel cell stack 100 to generate power (i.e., a power generation request P_req) (step S1). Next, the CPU 610 determines a pump drive command value VF_and for the flow control valve 406 based on the power generation request P_req obtained in step S1 (step S2). The pump drive command value VF_and is a voltage value to be applied to the flow control valve 406, and corresponds to a target value of the amount of reforming fuel to be supplied to the reformer 400.

FIG. 11A is a view of a map in which the relationship between the power generation request P_req and the pump drive command value VF_and is stored. The map in FIG. 11A is the map 622 that is stored in the memory 620 in FIG. 1. The amount of hydrogen needed to generate power increases as the requested amount of generated power increases. As shown in FIG. 11A, the pump drive command value VF_and increases in proportion to the power generation request P_req. Incidentally, even if the power generation request P_req is zero, the pump drive command value VF_and will not be zero because reforming fuel is necessary to maintain the temperature of the fuel cell stack 100.

Next, the CPU 610 derives an amount of reforming water to be supplied to the reformer 400 (i.e., a reforming water supply amount QW_f), based on the pump drive command value VF_and determined in step S2 (step S3). This reforming water supply amount QW_f may be derived from the pump drive command value VF_and and the control target value of the S/C ratio. FIG. 11B is a view of a map in which the relationship between the pump drive command value VF_and and the reforming water supply amount QW_f when the S/C ratio is 2.5 is stored. The map in FIG. 11B is the map 623 that is stored in the memory 620 in FIG. 1.

Next, the CPU 610 obtains an anode pressure PRS_and from the pressure sensor 410 (step S4). Then the CPU 610 derives a reforming water supply amount QW_p based on the anode pressure PRS_and obtained in step S4 (step S5). The anode pressure is controlled to vaporize the reforming water, so the reforming water supply amount QW_p increases in proportion to the anode pressure PRS_and, as shown in FIG. 11C. The map in FIG. 11C is the map 624 that is stored in the memory 620 in FIG. 1.

Next, the CPU 610 determines whether a value obtained by subtracting the reforming water supply amount QW_f from the reforming water supply amount QW_p is less than a threshold value QW_ref (such as 0.8 cc/min) (step S6). If the determination in step S6 is no, the CPU 610 ends the routine in the flowchart. As a result, an erroneous correction in the small anode pressure fluctuation region due to an excessive supply of reforming water can be avoided.

If the determination in step S6 is yes, the CPU 610 calculates a standard deviation σVF_and of the pump drive command value VF_and (step S7). The standard deviation σVF_and may be calculated from approximately 50 samples, for example. Next, the CPU 610 determines whether the standard deviation σVF_and is less than a threshold value σVF_and_ref (such as 0.01 V) (step S8).

If the standard deviation σVF_and is less than the threshold value σVF_and_ref, it can be determined that the fuel cell stack 100 is operating in a steady state in which the amount of power generated by the fuel cell stack 100 is stable. If, on the other hand, the standard deviation σVF_and is equal to or greater than the threshold value σVF_and_ref, it can be determined that the fuel cell stack 100 is in a transient operating state in which the amount of power generated by the fuel cell stack 100 is changing.

Therefore, if the determination in step S8 is no, the CPU 610 ends the routine in the flowchart. As a result, an erroneous correction in a transient operating state can be avoided. If, on the other hand, the determination in step S8 is yes, the CPU 610 calculates a standard deviation σPRS_and of the anode pressure PRS_and (step S9). The standard deviation σPRS_and may be calculated from approximately 50 samples, for example.

Next, the CPU 610 determines whether the standard deviation σPRS_and is smaller than a threshold value σPRS_and_lwr (such as 0.04 kPa) (step S10). If the determination in step S10 is yes, the CPU 610 obtains a reforming water flowrate correction value QW_fb according to Expression (1) below (step S11). Incidentally, the term dfb in Expression (1) below may be 1/10,000 cc/min, for example.

QW _(—) fb=QW _(—) fb+dfb  (1)

Next, the CPU 610 corrects a target supply amount QW from the reforming water metering electromagnetic valve 512 according to Expression (2) below (step S12). Incidentally, the term “map (VF_and)” in Expression (2) below is the reforming water supply amount determined according to the pump drive command value VF_and. Then the CPU 610 ends the routine in the flowchart.

QW=map(VF_and)+QW _(—) fb  (2)

If the determination in step S10 is no, the CPU 610 determines whether the standard deviation σPRS_and is less than a threshold value σPRS_and_upr (step S13). The threshold value σPRS_and_upr is a value that is larger than the threshold value σPRS_and_lwr, and is 0.16 kPa, for example. If the determination in step S13 is yes, the CPU 610 obtains the reforming water flowrate correction value QW_fb according to Expression (3) below (step S14). Incidentally, the term “dfb” in Expression (3) below may be 1/10,000 cc/min, for example.

QW _(—) fb=QW _(—) fb−dfb  (3)

Then, the CPU 610 executes step S12 and ends the routine in the flowchart. Incidentally, if the determination in step S13 is no, the CPU 610 ends the routine in the flowchart. As a result, the current target supply amount QW from the reforming water metering electromagnetic valve 512 is maintained at an appropriate value.

According to this example embodiment, the reforming water supply amount can be corrected based on the anode pressure fluctuation amount. As a result, the amount of supplied reforming water can be optimally controlled without using an expensive detector such as a flow meter that detects the steam flowrate. Also, the detection accuracy of the anode pressure fluctuation amount improves because the reforming water supply amount is corrected based on the standard deviation of the anode pressure. Furthermore, the S/C ratio can be controlled to an appropriate value because the reforming water supply amount is corrected such that the standard deviation of the anode pressure falls within a predetermined range.

Also, when the difference between the reforming water supply amount derived from the pump drive command value for the flow control valve 406 and the reforming water supply amount derived from the anode pressure is large, the reforming water supply amount is not corrected, so an erroneous correction in the small anode pressure fluctuation region due to an excessive supply of reforming water can be avoided. Moreover, when the fluctuation amount of the pump drive command value for the flow control valve 406 is large, the reforming water supply amount is not corrected, so an erroneous correction in a transient state in which the fluctuation amount of the reforming amount of the reformer 400 is large (i.e., when the fuel cell stack 100 is in a transient operating state in which the amount of fluctuation in power generation by the fuel cell stack 100 is large) can be avoided.

Incidentally, in the flowchart in FIG. 10, the pump drive command value VF_and is used to determine the amount of fluctuation in the reforming amount of the reformer 400 (i.e., the amount of fluctuation in the power generated by the fuel cell stack 100), but the invention is not limited to this. For example, detection results of the generated power, the generated current, or the generated voltage or the like of the fuel cell stack 100 may also be used. Further, the control value or the detection results of the reforming fuel supply amount or the reforming water supply amount, or information obtained based on those detection results, may also be used.

Also, in the flowchart in FIG. 10, there is only one type of reforming water flowrate correction value QW_fb, but the invention is not limited to this. For example, as shown in FIG. 11D, a table with different reforming water flowrate correction values QW_fb according to the classification of the pump drive command value VF_and may also be prepared. Further, a table with different reforming water flowrate correction values QW_fb according to the classification of a control value or detection value (i.e., the generated power, the generated current, or the generated voltage of the fuel cell stack 100, or the control value or the detection value of the reforming fuel supply amount or the reforming water supply amount) related to the amount of fluctuation in the reforming amount of the reformer 400 (i.e., the fluctuation in the amount of power generated by the fuel cell stack 100) other than the pump drive command value VF_and may also be prepared. Also, both the threshold value σPRS_and_upr and the threshold value σPRS_and_lwr may also be prepared for each classification. In this case, fine control according to the amount of heat received by the reformer may be executed.

In the foregoing example embodiment, the reforming water metering electromagnetic valve 512 may correspond to supply amount regulating means for regulating the amount of supplied reforming water. Also, the pressure sensor 410 may correspond to pressure detecting means for detecting the gas pressure inside the reformer. Moreover, the control portion 600 may correspond to correcting means for correcting the amount of supplied reforming water, and may also correspond to setting means for setting the amount of reforming fuel to be supplied to the reformer.

Incidentally, the present invention may also be applied to any type of fuel cell, such as a polymer electrolyte fuel cell (PEFC), a solid oxide fuel cell (SOFC), or a molten carbonate fuel cell (MCFC) or the like.

While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the appended claims. 

1-20. (canceled)
 21. A fuel reforming system comprising: a reformer that produces fuel gas that includes hydrogen by reforming a reforming fuel by a steam reforming reaction using reforming water, and; a supply amount regulating unit that regulates an amount of the reforming water supplied to the reformer; a pressure detecting unit that detects a gas pressure in the reformer; and a correcting unit that corrects the amount of supplied reforming water with the supply amount regulating unit based on a fluctuation amount in the gas pressure detected by the pressure detecting unit, wherein the correcting unit corrects the amount of supplied reforming water with the supply amount regulating unit based on a standard deviation of the gas pressure detected by the pressure detecting unit.
 22. The fuel reforming system according to claim 21, wherein the correcting unit performs a correction that decreases the amount of supplied reforming water, when the standard deviation of the gas pressure exceeds a first predetermined value.
 23. The fuel reforming system according to claim 21, wherein the correcting unit performs a correction that increases the amount of supplied reforming water, when the standard deviation of the gas pressure falls below a second predetermined value.
 24. The fuel reforming system according to claim 21, wherein the correcting unit performs a correction that decreases the amount of supplied reforming water, when the standard deviation of the gas pressure exceeds a first predetermined value, or the correcting unit performs a correction that increases the amount of supplied reforming water, when the standard deviation of the gas pressure falls below a second predetermined value; and the first predetermined value is larger than the second predetermined value.
 25. The fuel reforming system according to claim 24, wherein the first predetermined value and the second predetermined value are determined by a ratio between a number of moles of the reforming water supplied to the reformer and a number of moles of carbon in the reforming fuel supplied to the reformer.
 26. The fuel reforming system according to claim 25, wherein the ratio of the number of moles of the reforming water to the number of moles of the carbon of the first predetermined value is 3:1; and the ratio of the number of moles of the reforming water to the number of moles of the carbon of the second predetermined value is 2:1.
 27. The fuel reforming system according to claim 24, wherein the correcting unit does not correct the amount of supplied reforming water when the standard deviation of the gas pressure does not exceed the first predetermined value, and the standard deviation of the gas pressure does not fall below the second predetermined value.
 28. The fuel reforming system according to claim 21, wherein the correcting unit does not correct the amount of supplied reforming water with the supply amount regulating unit when the amount of fluctuation in a reforming amount of the reformer exceeds a third predetermined value.
 29. The fuel reforming system according to claim 21, further comprising a setting unit that sets an amount of reforming fuel supplied to the reformer, wherein the correcting unit does not correct the amount of supplied reforming water with the supply amount regulating unit when a difference between a reforming water supply amount derived based on the amount of supplied reforming fuel set by the setting unit and a reforming water supply amount derived based on the gas pressure detected by the pressure detecting unit exceeds a fourth predetermined value.
 30. The fuel reforming system according to claim 21, wherein a correction range of the reforming water supply amount corrected by the correcting unit is changed according to a reforming amount of the reformer.
 31. A control method of a fuel reforming system that regulates an amount of reforming water supplied to a reformer that produces fuel gas that includes hydrogen by reforming a reforming fuel by a steam reforming reaction using the reforming water, the control method comprising: detecting a gas pressure in the reformer; and correcting the amount of supplied reforming water based on a fluctuation amount in the detected gas pressure, wherein the amount of supplied reforming water is corrected based on a standard deviation of the detected gas pressure.
 32. The control method according to claim 31, wherein a correction that decreases the amount of supplied reforming water is performed when the standard deviation of the gas pressure exceeds a first predetermined value.
 33. The control method according to claim 31, wherein a correction that increases the amount of supplied reforming water is performed when the standard deviation of the gas pressure falls below a second predetermined value.
 34. The control method according to claim 31, wherein: a correction that decreases the amount of supplied reforming water is performed when the standard deviation of the gas pressure exceeds a first predetermined value, or a correction that increases the amount of supplied reforming water is performed when the standard deviation of the gas pressure falls below a second predetermined value; and the first predetermined value is larger than the second predetermined value.
 35. The control method according to claim 34, wherein the first predetermined value and the second predetermined value are determined by a ratio between a number of moles of the reforming water supplied to the reformer and a number of moles of carbon in the reforming fuel supplied to the reformer.
 36. The control method according to claim 35, wherein: the ratio of the number of moles of the reforming water to the number of moles of the carbon of the first predetermined value is 3:1; and the ratio of the number of moles of the reforming water to the number of moles of the carbon of the second predetermined value is 2:1.
 37. The control method according to claim 34, wherein the amount of supplied reforming water is not corrected when the standard deviation of the gas pressure does not exceed the first predetermined value, and the standard deviation of the gas pressure does not fall below the second predetermined value.
 38. The control method according to claim 31, wherein the amount of supplied reforming water is not corrected when the amount of fluctuation in a reforming amount of the reformer exceeds a third predetermined value.
 39. The control method according to claim 31, further comprising setting an amount of reforming fuel supplied to the reformer, wherein the amount of supplied reforming water is not corrected when a difference between a reforming water supply amount derived based on the set amount of supplied reforming fuel and a reforming water supply amount derived based on the detected gas pressure exceeds a fourth predetermined value.
 40. The control method according to claim 31, wherein a correction range of the corrected reforming water supply amount is changed according to a reforming amount of the reformer. 